What happens when a Transformer Trips on Differential and Buchholz Protection on a Heavy 22 kV Feeder Fault and how to analyze the Fault ?
Lets say a Transformer Tripped on Differential and Buchholz Protection on a Heavy 22 kV Feeder Fault.
Lets say the Magnitude of the 22 kV Feeder Fault is If R Phase= 6928A, If Y Phase= 7376 A, and If B Phase= 7944 A and If Neutral= 6100A.
So the 22 kV Feeder will Trip on OC & EF Highset in say around 100 ms.
So the Transformer will be feeding the Fault for this time (100ms).
During this time an Internal Fault may develop in the Transformer, causing it to Trip on Differential and Buchholz Protection.
How to Analyze such kinds of Faults?
Step 1: Flame Test
If the gas vented from the Main Buchholz burns vociferously with orange flame, it indicates the internal Short Circuit of high energy involving oil and Insulation.
Step 2: Main tank Oil sample & DGA test:
In the DGA test we need to check if the Proportion of Total Dissolved Gases in the oil are within limits (686 ppm) except,CH4 & C2H6 gas.
The heavy presence of Acetylene points to Discharge of High Energy with severe arcing and continuous power follow through.
Step 3: HV 10 kV Excitation & LV LV 500 V Excitation:
In this test 10 kV, 50 Hz AC is given to HV Winding RN, YN & BN.
Each Phase should Withstand the Volatge.
500 V, 50 Hz AC is given to LV side.
It should also withstand the Voltage.
If it does not Withstand the Voltage, it the indicates that the Insulation between the Winding and the Main Tank is weakened.
SFRA- Severe dislocation & winding deformation noticed in HV, LV R windings & Core-Coil Assembly.
